Ceramic tiles are commonly formed by pressing material in powder form, of between 1% and 10% moisture content, within a mold.
This forming method is commonly known as dry forming.
The soft material is loaded into the mold by known means.
After the mold has been closed by punches operated by pressing members, the powder undergoes an initial light pressing, with consequent volume reduction, to facilitate powder deaeration.
The initial light pressing, also known as the first pressing, is followed by the deaeration stage, during which pressing is interrupted and the mold is sometimes reopened to allow the air to escape.
This is followed by the main pressing to a pressure of about 400 kg/cm.sup.2, which ensures perfect powder compaction.
The main pressing generally takes place in several successive steps at increasing pressure up to a maximum pressure.
The pressing force exerted by the upper cross-member of the press is distributed over the total surface of the tiles pressed during each cycle.
It should be noted that each time reference is made to the term "pressure" in the text, this unless otherwise specified means the compacting pressure to which the powder is subjected within the forming mold.
The largest currently available presses have a capacity (pressing force) of 4000 tonnes, and during each cycle are able to press a surface area of not exceeding 10,000 cm.sup.2, where for example they can operate a die having three impressions of 54 cm.times.54 cm.
Achieving powder densification sufficient to ensure good quality of the finished product as the tile size increases requires an ever increasing press pressing force, implying presses of ever greater dimensions.
The object of this patent is to increase the powder densifying effect within the tile forming mold without increasing the press pressing force.
A method for pressing ceramic powder tending to achieve the aforesaid result is known, comprising subjecting to repeated blows the press cross-member with the upper part of the mold which exerts the pressing force on the powder.
This method, described in the patent application in the name of the present applicant No. 95A000063 of Oct. 18, 1995, has the advantage over conventional static pressing that for equal pressing force exerted by the press a greater powder densification is obtained, i.e., a higher powder density.
The known method has however a certain number of drawbacks which have prevented its implementation on an industrial scale.
In this respect, the effects produced by the blows cannot in practice be kept under valid control because secondary factors intervene such as friction resistance, play between the moving parts, inertia and other phenomena, which modify the system parameters.
The impossibility of maintaining the system parameters under control has resulted in vibration being partly transmitted to the machine structure, with problems of excessive noise, loosening of members or breakages occurring.
Moreover, to be able to apply the method successfully and with easily available means, the punch and the press cross-member must be of relatively small mass and hence dimensions.
The object of the invention is therefore to eliminate said drawbacks of the known method.
The object is attained, according to the invention, by subjecting the powder mass to be compacted both to the press pressing force and simultaneously to vibrations which are limited substantially to the powder mass without involving the pressing members.
This is in accordance with the pressing method and device defined in the claims.
The merits, the construction and operational characteristics of the invention will be more apparent from the description given hereinafter with reference to the accompanying drawings, which show a preferred embodiment thereof.